.... I had scarce entered the fort second from the river when a frightful explosion took place, where a group of men were standing in the quadrangle. One of the New-York Thirty-eighth (which regiment, Col. J. H. Ward , first occupied this strong-hold) men had trodden on the spring of an infernal rebel machine. Two soldiers were killed, I think, and others wounded. Just afterward the McClellan dragoons came on,...